18 Inch Rear Tire Differences?

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Hi all, so I've got a question about a rear tire choice. On my dirtbike it has a 21" front tire and 18" rear tire. On Motorcycle Superstore they have several different 18" rear tires in the specific tire that I want.

What do the numbers mean? Obviously the 18 is for the tire size... but what does the 100/100, 100/90, 110/100, 120/90 mean? Is it something to do with like sidewall construction, I really don't know. Could someone clarify this with me please?